Bone density scanning, also called dual-energy X-ray absorptiometry (DEXA) or bone densitometry, is an enhanced form of X-ray technology that is used to measure bone loss. It can also be used to determine body composition, as it can measure the percentage of the body’s lean muscle and fat. The most common purpose, however, is to evaluate if a person’s bones are weak, likely to fracture, or if they have osteoporosis.
When determining if a patient has a low bone density or the condition is becoming worse, a DEXA scan is generally more accurate than an ordinary X-ray, since it can spot even minor changes in bone loss. This DEXA scan is most often performed on the lower spine and hips whereby low-energy x-rays are directed through the bone to measure its mineral density. Mineral density is an accurate predictor of bone strength. The results of this test are commonly used to diagnose osteoporosis and to assess an individual’s risk for developing fractures.
How is a DEXA scan performed?
DEXA is the short form of Dual Energy X-ray Absorptiometry. Its apparatus consists of a flat padded table, a movable arm fitted above to hold the X-ray detector, and a device below the table that generates X-rays. The technician positions the patient on the table and may put a wedge under their knees to help flatten their spine for the picture, or to help position their hip correctly. The arms are often also positioned for scanning. The patient is asked to hold very still while the imaging arm above gradually passes across their body. The level of X-ray radiation is low enough to let the technician stay in the room with the patient while the scan is performed. The whole procedure takes only a few minutes.
DEXA scan details (before/during/after)
A person can eat and drink usually on the day of the DEXA procedure. However, if taking calcium supplements, it is recommended to stop taking them at least 24 hours before the DEXA scan. Prior to the exam, patients are given a gown and asked to remove all metal objects such as jewelry, belts or underwired garments, and eyeglasses.
During the DEXA procedure, the imaging arm moves gradually over the patient’s body while a low-dose energy beam passes through. The technician usually scans the hips and spine as these are the common sites for fractures in osteoporosis. In certain cases, the finger, wrist, and lower arm may also be scanned. A vertebral fracture evaluation may also be conducted during a DEXA to determine if there is any vertebral fracture risk.
The results of a bone density DEXA scan use a system called a T-score that is based on comparing the person’s scan with the bone density of a healthy young adult of the same sex. World Health Organization (WHO) has prescribed the following T-score as per the bone density levels:
- A T-score of -1.0 or higher means the bone density is normal.
- A T-score of -1.1 to -2.4 means osteopenia or bone density is low.
- A T-score of -2.5 or lower means osteoporosis exists.
A DEXA scan also often uses the Z-score that shows the amount of bone a patient has in comparison to other people of the same size, sex, and age. According to the International Society for Clinical Densitometry, a Z-score above -2.0 is normal, and a score below -2.0 is considered below the normal range.
As bone changes occur comparatively slowly, based on the patient’s initial test results, the patient may not be required to test for at least two years. However, if the health care provider observes that the patient may lose bone rapidly, the patient may need to get retested sooner.